Doppler radar systems operating at Industrial, Scientific, and Medical (ISM) bands near 2.4 GHz, 5.8 GHz, and 24 GHz have been proven to be effective in non-contact vital sign and vibration detection. Recent research has shown the use of higher radar frequencies (shorter wavelength λ) near 60-GHz ISM band provide a larger system demodulation gain to distinguish small displacements at a longer distance. The use of such higher radar frequencies also helps achieve a highly compact and integrated system, as the antenna size and component sizes are reduced. However, the respiratory chest-wall movement (mr≈1-6 mm) comparable to the wavelength at 60 GHz (5 mm) results in strong nonlinear Doppler phase modulation, which introduces numerous harmonics and inter-modulation peaks on the spectrum. In addition, the heartbeat signal, having an amplitude (mh≈0.2 mm) that is one order of magnitude smaller than that the amplitude of the respiration signal, is usually overwhelmed by the harmonics and noise. Most of the existing analysis on, and techniques for accommodating for, Doppler radar nonlinearity is based on Bessel functions and based on the assumption that the target movement is one or more simple sinusoidal movements, and is usually susceptible to system noise and non-ideality.